7T MRA for Distinguishing Small Intracranial Aneurysms from Variant Anatomy: Protocols and Impact

Unruptured intracranial aneurysms are increasingly detected on noninvasive imaging, but false-positives from limited resolution can lead to unnecessary anxiety, follow-up, and invasive procedures. We investigated multiple 7T MRA sequences for their ability to reduce aneurysm overdiagnosis by differentiating them from variant anatomy. We also evaluated which characteristics of suspected aneurysms were associated with a greater likelihood of diagnostic reversal by 7T MRA and estimated the resulting impact on imaging use and cost.

In this retrospective study, 41 suspected aneurysms in 34 patients who underwent 7T MRA during a 22-month period were evaluated using 3 sequences: conventional TOF, a compressed sensing version of TOF with improved spatial resolution, and contrast-enhanced MRA. Patient demographics, aneurysm size, and prior imaging technique were recorded. Two neuroradiologists assessed each lesion for reclassification as an anatomic variant on the basis of the 7T appearance. Logistic regression was used to identify any significant relationships between the 7T sequence type or aneurysm characteristics and the likelihood of downgrade.

Overall, 7T MRA permitted a diagnostic downgrade in 46% of suspected aneurysms. Downgrade rates were 30% for conventional TOF, 41% for compressed sensing TOF, and 39% for contrast-enhanced MRA, with no single sequence proving statistically superior. Lesions detected on 1.5T MRA were significantly more likely to be downgraded compared with those found with 3T MRA (53% versus 38%, P < .05, OR, 2.53). Additionally, aneurysm size was significantly inversely related to downgrade likelihood, with all lesions <1 mm and 63% of lesions 1-2 mm being reclassified, whereas none of the lesions of >3 mm were downgraded (P < .001, OR, 0.30 per millimeter increase in size; 95% CI, 0.15-0.58). On the basis of these findings, we estimate that 7T MRA can reduce unnecessary surveillance by up to 2.08 scans per patient-resulting in cost savings of up to $1388 per patient, depending on the surveillance technique used and assuming the federal reimbursement rate.

7T MRA frequently reclassifies small, suspected aneurysms as anatomic variants, especially in cases identified by lower-field-strength imaging and in smaller lesions. The associated potential for reducing unnecessary follow-up imaging has important cost-saving implications.